I'm not really surprised at all. McLaren have fantastic resources behind then and the fact that they have had a considerable budget spike this year due to lots of sponcership from Spanish companies, courtesy of their Alonso signing, it looks like they have the budget to run the wind tunnel 24/7.
However, its not just this, its also due to changes in F1 have come back to favouring their design again - Autosport pointed out in the Christmas issue. For the past few seasons McLarens have been fairly neutral in terms of their balance, so they tend to spread the load quite evenly across all the tires. If you remember two years back they suddenly found competetiveness again - this was due the rules that year not allowing tire changes during the race and so both tire manufacturers basically had to make tires that wore evenly from the fronts to the rears (which also results in a wider front tire), something that suits the McLaren balance very much, but less so the Renault's which are very torque'y cars with a heavy rearward bias. In '06 the change to the rules to allow tire changes again meant that the all round wear wasnt an issue so Michelin went back to a mix that utimately favoured Renault again. Now that Michelin are out of the picture and Bridgestone are basically providing the "control" tires they are producing large front tires, with a more even wear rate - again, playing to balance naturally favoured by McLarens design (and we can see that its absolutely killing Renault and their rearward bias now).
This was actually one of the main concerns when Alonso came to McLaren, that he wouldn't be able to get used to the balance of the chassis. It looked like it wasn't going to be an issue for him through much of testing, however on the strength of the performances on Sunday you may begin to wonder if the balance may begin to favour Hamilton more... (Damn, he's going to be interesting to watch in Europe, when he's on very familiar tracks).